The present disclosure relates generally to Pulse Width Modulation (PWM) controllers and more particularly to single-ended primary-inductance converters (SEPICs) including frequency switching functions for a PWM control signal.
(PWM) is widely used to control switch mode power supplies, such as the power supplies that are found in automotive systems. A typical approach used to generate PWM control signals uses a flip-flop, comparator and a ramp generator. At the beginning of each ramp up from the ramp generator, the output of the flip-flop is set to on, resulting in a high voltage output. The comparator resets the flip-flop to off when the output of the ramp generator exceeds a predefined threshold (when the ramp up exceeds the threshold). The threshold is defined by an error amplifier that is part of a feedback control loop within the control system or defined in a controller. This process repeats at a fixed frequency generating a square wave output from the flip-flop. The square wave output functions as the PWM control signal.
In some instances, the PWM frequency is adjusted to compensate for operating conditions of a DC/DC converter, such as a SEPIC, used as part of the aforementioned ramp generator. Operating conditions that can require this adjustment are sudden changes to the input voltage of the DC/DC converter, the output voltage of the DC/DC converter, a connected load, or any other similar operating condition.
One type of DC/DC converter that is frequently utilized in PWM systems is a single-ended primary-inductor converter (alternately referred to as a SEPIC). PWM SEPICs typically include a compensation loop design that keeps the system stable when the converter is operating in a discontinuous conduction mode. When the input voltage to the converter decreases below a predefined threshold, the operating frequency of the converter is decreased in order to keep a power stage of the SEPIC stable. If the peak and valley values of the sawtooth signal remain the same (i.e. the slope of the sawtooth is adjusted proportional to the ratio of frequencies) then the operating frequency change causes an overshoot or undershoot. Similarly, when the frequency is increased after the input voltage exceeds the predefined threshold, there is a corresponding undershoot at the SEPIC output voltage.